Use windows openssh agent by default

[PowerShell/openssh-portable](https://github.com/PowerShell/openssh-portable) creates a ssh agent socket `SSH_AUTH_SOCK=\\.\pipe\openssh-ssh-agent` (see [How to run ssh-add on windows? - Stack Overflow](https://stackoverflow.com/questions/18683092/how-to-run-ssh-add-on-windows/68386656#68386656)). I have tried manually specify the environment variable `%SSH_AUTH_SOCK%` to be `\\.\pipe\openssh-ssh-agent`. It worked. I think it is reasonable to set this environment variable by default, or we can provide an option to set this environment variable.